6.1 TI Precision Labs - Op Amps: Slew Rate 1

Hello, and welcome to the TI precision lab discussing op amp slew rate, Part 1. In this video, we'll go over the theory behind slew rate and compare the slew rate and current consumption of different TI amplifiers.
Slew rate is defined as the maximum rate of change of an op amps output voltage, and is given in units of volts per microsecond. Slew rate is measured by applying a large signal step, such as one volt, to the input of the op amp, and measuring the rate of change from 10% to 90% of the output signal's amplitude.
The data sheet large signal step response is an indication of the amplifier slew rate. In this example, we calculate the slew rate to be about 29 volts per microsecond. Again, the slew rate definition only considers the rate of change of the signal from 10% to 90%, which in this case is 1 volt to 9 volts.
Slew rate is a different specification than small signal bandwidth, which considers differential input signals of plus or minus 100 millivolts or less.
Before we get into an in-depth slew rate discussion, let's first review some basics. The equation that defines how a capacitor works states that the current flow through a capacitor is equal to the capacitance times the derivative of voltage with respect to time. This behavior can also be interpreted to mean that if you have a constant current, then the voltage across the capacitor will rise linearly over time.
This is important with respect to slew rate of an amplifier. An amplifier has an internal GM, or transconductant stage, which takes the input differential voltage and converts it to an output current, I out.
I out flows into the next stage, where it is used to charge C sub c, which is called the Miller capacitance. If I out is a constant, then the voltage across C sub c will rise linearly with time, just like we discussed on the previous slide.
For slow-moving signals, I out is less than some maximum value I out max. This means that I out is able to change according to the differential input voltage without being limited.
But for rapidly-moving large signals, I out reaches its maximum and becomes limited to some constant value. In this case, the input to the amplifier will no longer be a virtual short. And therefore, a differential voltage will develop across the input pins.
Since I out is a constant, V out across the Miller capacitor C sub c rises linearly over time. This is when the output of the amplifier is considered to be slew rate limited, which is the fastest that the output voltage can change.
Here is a transistor-level view of what's happening inside the amplifier when we apply a step input to the amplifier, which is an extremely fast-moving signal. One transistor in the GM stage will be turned off. And the other will be turned fully on. The current flowing through the transistor which is on is the I out max mentioned in the previous slide. As previously discussed, I out max flows into the Miller capacitor C sub c, causing the output voltage to ramp linearly over time.
Here, we compare the typical slew rate and quiescent current, or IQ, for different amplifiers. On one end of the spectrum, we have the OPA369, which is a very low IQ and low slew rate device. For 0.8 microamps of current, we can achieve around five millivolts per microsecond of slew. Compare that to the OPA847, which consumes 18.1 milliamps of IQ, but can slew at 850 volts per microsecond. This shows us the amplifiers with higher slew rate and, therefore, higher bandwidth, tend to have higher current consumption.
We can easily simulate slew rate using TINA TI. Simply apply a step function to the input of the amplifier, which, in this case, is a plus or minus 1-volt square wave. You can see that when this input step is applied, the input offset voltage changes from 0 volts, which indicates a virtual short, to some other voltage-- around 900 millivolts, in this case.
Most importantly, the output voltage becomes slew rate limited, shown as a constant ramp in voltage over time, until finally reaching its true value.
You can observe the input offset voltage moving linearly back to 0 volts as well.
Calculating the slew rate from this plot gives a result of 0.795 volts per microsecond. The data sheet for this device the OPA2188, lists the slew rate as 0.8 volts per microsecond, indicating that the model accurately simulates the slew rate of the amplifier.
This slide emphasizes the fact that we no longer have a virtual short whenever a step function is applied to the input of the amplifier. The output moves slower than the input signal. And so we have some finite voltage across the input pins. As the output ramps linearly to its final value, the input gets closer and closer to a virtual short again. And once it does, the amplifier returns to its closed loop configuration.
An amplifier's data sheet will provide a plot showing slew rate versus temperature, often for both positive and negative slew rates. Positive slew rate write occurs when a signal is rising. And negative slew rate occurs when a signal is falling. Typically, the slew rate of an amplifier will increase with increasing temperature.
Some amplifiers include a slew boost circuit, which allows for faster slew rates. An example of an amplifier with slew boost is shown in this large signal step response plot. What happens is that the device has two different slew rates-- an initial rate, which is very fast, and a second slower rate as the output settles to its final value.
You may ask yourself, well, why doesn't the amplifier just have one slew rate which is always fast? The reason is that with one extremely fast slew rate, the output would have a large overshoot. When that overshoot occurred, the amplifier would try to compensate for this. And the negative slew would kick in, resulting in a large negative overshoot. This behavior would continue, resulting in oscillation.
So how does a slew boost look compared to a standard amplifier? On the left-hand side is the response of a standard amplifier. The green region shows the small signal response when the differential input voltage is less than plus or minus 100 millivolts, where the amplifier can linearly change the current flowing into the Miller capacitance. The blue region shows the large signal response where the differential voltage is greater than plus or minus 100 millivolts, where the amplifier reaches its slew limit and the current flow into the Miller capacitance is held constant.
We have a similar situation for an amplifier with slew boost. There is still a small signal response shown by the green region. But once the differential input voltage exceeds certain value, we reach the slew rate limit, indicated by the blue region, and eventually the slew boost, indicated by the red region. Therefore, when a large step function is applied to the input of the amplifier, the device will initially see a large differential input voltage and will be in slew boost mode, allowing a large output current into the Miller capacitance, and, therefore, a quickly ramping output voltage. As the differential input voltage decreases, the amplifier will move to its standard slew rate, and finally to its small signal response once the input voltage becomes small enough. At this point, the output will settle, and the inputs of the amplifier will once again be a virtual short.
So far, we have considered square waves when looking at slew rate. However, slew rate can limit or distort any signal amplified by an op amp. This characteristic of the op amp is called the full power bandwidth, our maximum output voltage versus frequency.
This graph shows the maximum sinusoidal waveform that can be output without running into slew-induced distortion. In this example, we have a 200 kilohertz signal at both 7.5 volts peak and 10 volts peak. At 10 volts peak, the output signal is above the curve, and will be distorted by slew rate limits. At 7.5 volts peak, the output signal is under the curve, and therefore will not be distorted by slew rate limitations.
This simulation verifies that to the 200-kilohertz signal with 7.5 volts peak output amplitude will not be distorted. The output looks like an accurately amplified version of the input.
As a side note, the offset voltage looks like a sinusoidal wave, also. This offset is really just the output voltage divided by AOL at the frequency of interest, which, in this case, is 200 kilohertz.
This example verifies that the signal with 10 volts peak will be distorted. The output looks more like a triangle wave than a sinusoidal wave, due to the slew-induced distortion. The input offset is also clearly distorted. And it's no longer equal to the output voltage divided by AOL.
Finally, this slide illustrates that the maximum output versus frequency curve can be derived with calculus. You can go through the math on your own. But the key point is that the final equation, where V peak equals the slew rate divided by 2 pi f, can be used if this curve is not available. The example shown in red confirms that the equation yields the same result as the curve.
That concludes this video on slew rate. Thank you for watching. Please try the quiz to check your understanding of this video's content. 大家好，欢迎来到 TI 高精度实验室，本视频将介绍 运算放大器转换速率第 1 部分。 在本视频中，我们将回顾 转换速率背后的理论， 并比较不同 TI 放大器的 转换速率和电流消耗。 转换速率 被定义为 运算放大器 输出电压的 最大变化率， 以 V/us 为单位。 测量转换速率时， 可以在运输放大器的 输入端施加 一个大信号步长， 如 1 V/V，然后测量 输出端信号幅值 从 10% 增至 90% 的变化率。 数据表中给出的 大信号阶跃响应 就是放大器转换 速率的一个指标。 在本示例中， 我们计算出 转换速率 约为 29 V/us。 同样，转换速率 定义仅考虑 信号从 10% 增至 90% 的变化率， 这在本例中指 从 1 V 增至 9 V 的变化率。 转换速率与 小信号带宽 是不同的规格， 后者考虑 ±100 mV 或更小的 差分输入信号。 在我们深入探讨 转换速率之前， 让我们先来复习一些基础知识。 该等式定义 电容器的工作原理， 描述的是 流经电容器的 电流等于电容 乘以电压随时间的 变化率。 这种行为 也可以理解为， 当电流恒定时， 电容器两端的 电压将会 随着时间 呈线性增加。 这对放大器的 转换速率很重要。 放大器有一个内部 GM，或称跨导级， 它会把 输入差分电压 转换为输出 电流 Iout。 Iout 流入 下一级， 并在此被用于 对米勒电容 Cc 进行充电。 当 Iout 是常数时， Cc 两端的电压 将会随着时间 呈线性增加， 就像我们在上一张 幻灯片中讨论的那样。 对于缓慢变化的信号， Iout 小于某个最大值 Iout(max)。 这意味着 Iout 能够 不受限制地 随着差分输入电压 而变化。 但对于快速 变化的大信号， Iout 将达到其 最大值， 且会被局限于某个定值。 在这种情况下， 放大器的输入端 将不再是 虚短路。 因此， 输入引脚 两端将会出现 差分电压。 由于 Iout 是常数， 因而米勒电容器 Cc 两端的 Vout 将会随着时间呈线性增加。 此时就认为 放大器的输出 达到转换速率 极限，即输出电压的 变化速度 达到最大值。 这是当我们向放大器 施加一个步长输入， 即一个变化速度 极快的信号时， 放大器内部 晶体管级 发生的情况。 GM 级中的一个 晶体管将会被截止， 另一个晶体管 将被完全导通。 流经导通 晶体管的电流 就是在上一张幻灯片中 提及的 Iout(max)。 像之前讨论的一样， Iout(max) 流入 米勒 电容器 Cc， 导致输入电压随着 时间呈线性增加。 现在，我们比较 不同放大器的 典型转换速率与 静态电流 IQ。 在表格的 第一行中可见 OPA369 是一个 IQ 极低 且转换速率很低的器件。 静态电流 为 0.8 uA， 转换速率 为 5 mV/us。 我们将其与 OPA847 进行比较。 OPA847 消耗的 IQ 为 18.1 mA， 但转换速率为 850 V/us。 这说明放大器 转换速率越高， 带宽更大， 其消耗的 电流往往 也会越大。 我们可以使用 TINA TI 来轻松地仿真转换速率。 只需在放大器 输入端应用一个 阶跃函数即可， 在本例中， 这是 1 V 或 -1 V 的方波。 您可以看到， 当施加此输入步长时， 输入偏移电压 从表明虚短路的 0 V 变为 另一个电压， 在本例中，约为 900 mV。 更重要的是， 输出电压 达到了转换速率极限， 即电压随着时间 以恒定斜率变化， 直至最终达到其真值。 您可以观察到， 输入偏移电压 也线性变回 0 V。 从此图计算 转换速率 可以得到 0.795 V/us 的结果。 该器件 OPA2188 的 数据表列出的 转换速率为 0.8 V/us， 表明模型 准确地仿真了 放大器的转换速率。 此幻灯片着重说明， 当阶跃函数 被应用至 放大器输入端时， 输入端不再是虚短路。 输出信号的变化速度 比输入信号慢。 因此，输入引脚两端 存在某个有限电压。 当输出以线性形式 到达其最终值， 输入会再次变得 愈发接近虚短路。 一旦变为虚短路， 放大器会恢复 其闭环配置。 放大器的 数据表会给出 转换速率与 温度的关系图， 通常会给出正转换速率 和负转换速率。 正转换速率在 信号上升时出现。 负转换速率在 信号下降时出现。 一般而言，放大器的 转换速率 会随温度的 上升而增大。 有些放大器包含 一个转换增强电路， 这可实现更高的转换速率。 此大信号阶跃响应 图中给出了一个 具有转换增强功能的 放大器示例。 可以看到，器件具有 两个不同的转换速率， 一个是非常快的 初始转换速率， 另一个是当输出逐渐 稳定到其最终值时的 较慢的第二阶段转换速率。 您可能会好奇， 放大器为什么 不能只有一个 始终很快的转换速率？ 原因在于，如果放大器 只有一个极快的转换速率， 则输出将会有 一个很大的过冲。 当该过冲出现时， 放大器会 尝试对其 进行补偿。 这时负转换速率 会开始起作用， 从而导致 较大的负过冲。 这种行为会持续下去， 导致振荡发生。 那么具有转换增强功能的 放大器与标准放大器相比 有何不同呢？ 左侧是标准 放大器的 响应。 绿色区域 显示了 差分输入 电压小于 ±100 mV 时的 小信号响应， 这时放大器可以 线性更改流入 米勒电容的电流。 蓝色区域 显示了 差分输入 电压大于 ±100 mV 时的 大信号响应， 这时放大器达到了 其转换速率极限， 并且流入 米勒电容的 电流保持不变。 对于具有转换增强功能的 放大器而言，情况与之相似。 绿色区域 仍然显示了 小信号响应。 但当差分输入电压 超过某一特定值时， 放大器达到转换速率极限， 如蓝色区域所示， 最终转换增强功能起作用， 如红色区域所示。 因此，当给 放大器的输入端 应用一个 大阶跃函数时， 器件会首先判断 差分输入电压的大小， 如果很大，则会进入 转换增强模式， 使得大输入电流 能够流入米勒电容， 从而快速增大 输出电压。 当差分输入 电压降低时， 放大器会进入其 标准转换速率模式， 一旦输入电压 变得足够小， 放大器就会进行 小信号响应。 此时，输出将会 稳定下来， 而放大器的输入端 会再次变为 虚短路。 到目前为止，我们在 研究转换速率时， 只考虑了方波。 不过，转换速率可以 限制运算放大器 放大的任何信号 或导致其失真。 运算放大器的 这种特性 被称为全功率带宽， 它描述了最大输出电压 与频率的关系。 这幅图显示了 可以作为输出 且无转换 导致的失真的 最大正弦波形。 在本示例中， 我们在 7.5 Vpk 和 10 Vpk 处有 200 KHz 信号。 在 10 Vpk 处，输出信号 位于曲线上方， 将会受到转换速率限制 造成的失真影响。 在 7.5 Vpk 处，输出信号 位于曲线下方， 因此不会受到 转换速率限制 造成的失真影响。 该仿真证实 具有 7.5 Vpk 输出幅值的 200 KHz 信号 将不会失真。 输出看起来像 精确放大的 输入版本。 此外， 偏移电压 看起来也像 正弦波。 该偏移实际上就是 输出电压除以 所需频率处的 AOL， 这在本例中 是 200 KHz。 该示例证实 具有 10 Vpk 幅值的 信号将会失真。 输出看起来更像 三角波而不像 正弦波，原因在于 转换导致的失真。 输入偏移 也明显失真。 它不再等于 输出电压除以 AOL。 最后，此幻灯片 说明最大输出 与频率的关系曲线 可使用微积分推导出来。 您可以自己 进行数学计算。 但关键点在于， 如果该曲线不可用， 可以使用 最终等式， 其中 Vpk 等于 转换速率除以 2πf。 红色框中显示的 示例证明等式 得出的结果 与曲线相符。 本段关于转换速率的 视频到此结束。 谢谢观看。 请尝试完成测验以 检查您对本视频 内容的理解。

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Date:
March 23, 2015

This is the first of four videos in the TI Precision Labs – Op Amps curriculum that addresses operational amplifier slew rate. In this training, we discuss the theory behind slew rate and compare the slew rate and current consumption of different TI amplifiers

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